Early programmable devices were one-time configurable. For example, configuration may have been achieved by “blowing”—i.e., opening—fusible links. Alternatively, the configuration may have been stored in a programmable read-only memory. Those devices generally provided the user with the ability to configure the devices for “sum-of-products” (or “P-TERM”) logic operations. Later, such programmable logic devices incorporating erasable programmable read-only memory (EPROM) for configuration became available, allowing the devices to be reconfigured.
Still later, programmable devices incorporating static random access memory (SRAM) elements for configuration became available. These devices, which also can be reconfigured, store their configuration in a nonvolatile memory such as an EPROM, from which the configuration is loaded into the SRAM elements when the device is powered up. These devices generally provide the user with the ability to configure the devices for look-up-table-type logic operations.
At some point, such devices began to be provided with embedded blocks of random access memory that could be configured by the user to act as random access memory, read-only memory, or logic (such as P-TERM logic). Moreover, as programmable devices have become larger, it has become more common to add dedicated circuits on the programmable devices for various commonly-used functions. Such dedicated circuits could include phase-locked loops or delay-locked loops for clock generation, as well as various circuits for various mathematical operations such as addition or multiplication. This spares users from having to create equivalent circuits by configuring the available general-purpose programmable logic.
While it may have been possible to configure the earliest programmable logic devices manually, simply by determining mentally where various elements should be laid out, it was common even in connection with such earlier devices to provide programming software that allowed a user to lay out logic as desired and then translate that logic into a configuration for the programmable device. With current larger devices, including those with the aforementioned dedicated circuitry, it would be impractical to attempt to lay out the logic without such software. Such software also now commonly includes pre-defined functions, commonly referred to as “cores,” for configuring certain commonly-used structures, and particularly for configuring circuits for mathematical operations incorporating the aforementioned dedicated circuits. For example, cores may be provided for various trigonometric or algebraic functions.
Available programming software has heretofore assumed that when a user specifies a certain logic design, that design is static. However, in some cases, it may be that the design includes a function that is parameterizable or that may operate differently when its inputs are in different ranges. This is particularly the case where the programmable device is part of a larger heterogeneous system where some other device, such as a microprocessor, calls upon the programmable device to process data. Depending on the characteristics of those data, slightly different versions of the user logic design may be required.